A CO2 laser typically includes spaced apart parallel discharge-electrodes in an enclosure containing the lasing gas mixture. A laser resonator is configured with a longitudinal axis thereof extending between the electrodes. A gas discharge is struck (lit) in the lasing gas mixture by applying RF power usually in the form of RF voltage pulses to the discharge electrodes. This causes the laser resonator to deliver pulses of laser radiation corresponding in duration and frequency to the duration and frequency of the RF voltage pulse. When pulses are being delivered there is sufficient ionization remaining in the mixture following the application of one RF pulse that the next pulse essentially immediately re-lights the discharge for delivery the next pulse.
In a commercially-available pulsed CO2 lasers there is typically some means provided for maintaining some level of ionization in the lasing gas mixture when laser pulse trains are not being delivered. This is commonly referred to as pre-ionization. Pre-ionization facilitates lighting the gas discharge when it is desired to deliver laser pulses. Pre-ionization means are usually configured to minimize any delay between application of the RF pulse power to the electrodes and the delivery of laser pulses. The pre-ionization means should also be configured such that whatever minimum delay remains, that delay is predictable and repeatable.
In early low power lasers for example with less than 100 Watts (W) average power output pre-ionization has been provided by a separate pre-ionization device, not unlike a spark-plug, and operated by a power supply separate from the RF power supply for the discharge electrodes. This method was found to be inadequate for lasers with higher power output. A method referred to as a simmer discharge method has been developed for such lasers. In the simmer discharge method, pre-ionization is created by applying RF pulses, from the main RF power supply of the laser, to the discharge electrodes with a pulse-duration long enough to create free electrons and provide the required ionization, but not long enough to actually light a discharge (plasma) and cause laser action.
A challenge to the development of the simmer discharge method has been to find means of accommodating a difference in load impedance of the discharge that exists between the simmer discharge (pre-ionization) condition and the lit-discharge (lasing) condition. One such means is described in U.S. patent application Ser. No. 12/367,174, filed Feb. 6, 2009, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. A simmer discharge method in accordance with this description functions reliably in pulsed CO2 lasers having an average output power up to 400 W.
It was found that when attempting to implement this method in pulsed CO2 lasers having an average power output up to 1000 W the pre-ionization-pulse duration that had provided reliable consistent pre-ionization in the lower power lasers sporadically caused unwanted laser action in the higher power lasers. It was also found that because of statistical variations between nominally the same 1000 W lasers, a pre-ionization pulse duration that could provide pre-ionization without unwanted laser action was difficult to predict. This necessitated a time-consuming and costly “calibration” of each laser to determine a specific optimum pre-ionization pulse duration for that laser. Attempting to use a duration short enough to avoid lasing without such calibration led to unreliable discharge ignition. It was found necessary to further develop the pre-ionization method to avoid the conflict between avoiding unwanted laser action and unreliable discharge ignition.
An exemplary description of the operation of the above-discussed prior-art simmer discharge procedure in a gas discharge laser is set forth below with reference to FIGS. 1A and 1B, which each depict voltage as a function of time, and together provide a timing diagram. FIG. 1A illustrates timing of simmer pulses, and FIG. 1B illustrates timing of laser pulses. Before turning on the laser, the simmer discharge is first turned on at a time t0.
Simmer pulse generating circuitry commands an RF Power Supply (RFPS) to emit a RF simmer-pulse train consisting of short RF pulses exemplified in FIG. 1A by pulses SP1, SP2, and SP3. The RF pulses have a width (duration) WS, and a peak voltage V. The pulses are repeated with a time interval T therebetween, i.e., at a pulse-repetition frequency (PRF') equal to 1/T. In this approach the RFPS powers the unlit discharge, i.e., the simmer function, and the lit discharge. The peak voltage V is nominally the same for the lit and unlit discharge conditions. The duration of a simmer pulse is shorter than the duration of a laser pulse and too short to actually cause a lasing discharge. By way of example a simmer pulse may have a duration of about 4 μs. The simmer pulses precondition the laser gas during a laser warm up period by generating sufficient number of free electrons within the gas between the discharge electrodes. The initial warm up period can be as long as several minutes. The free electrons insure a discharge is quickly ignited when a user command to emit a laser pulse instructs the RFPS to emit an RF pulse having a width (duration) WL long enough to excite a lasing discharge and emit a laser pulse, for example about 50 microseconds or longer. The period T is selected such there will always be sufficient free electrons during this period to facilitate ignition of a lasing discharge when required.
In FIG. 1B the user command pulse is arbitrarily selected to arrive at time t2, temporally spaced by a duration tD following the termination of simmer pulse SP3 at time t1. As time tD is less than the time T between simmer pulses, there are sufficient free electrons in the discharge to promptly ignite the discharge with little time jitter (delay). On receipt of the laser command pulse the simmer pulse command circuitry is disabled. The laser pulse (LP1) is terminated at time t3. If another user command signal is not received before another period T has elapsed following t3, the simmer pulse circuitry is re-activated to cause simmer pulses to be delivered by the RFPS. In FIG. 1B, pulse SP4, represents the first of such pulses. The simmer circuitry commands the RFPS to deliver simmer pulses with period T therebetween until another user command is received to deliver a laser pulse.
The prior-art system described above works very well, but there is always a question about how long a simmer pulse should be for any given laser arrangement. Certainly the duration must excite the gas without causing lasing. Extensive experimentation with a 400 W CO2 slab laser has indicated that a 4 μs pulse as exemplified above satisfied these criteria. However, when the same simmer-pulse width was applied in a 1000 W CO2 slab laser, laser action occurred before the end of the simmer pulse and a small amount of laser power was emitted by the laser when it had not received a signal to do so. Reducing the simmer pulse width to 3 μs is in the 1000 W slab laser appeared to work acceptably, at least in that one particular laser. A problem is that there are statistical variations between lasers in the same model family so it can not be certain that a simmer pulse duration that does not cause lasing in one unit of the family will also not cause lasing in another unit of the family.
While an appropriate simmer-pulse duration for any particular laser can be determined relatively quickly by experiment, this experimental determination adds time and cost to the laser production. Accordingly there is a need for a method and circuitry for delivering simmer pulses that automatically avoids unwanted lasing during delivery of the simmer pulses.